The invention relates to a light curtain apparatus comprising a periodically operating light transmitter arrangement; a first strip-like concave mirror which extends over the width of the monitored region and which receives the light beam from the light transmitter arrangement from different directions and generates scanning beams therefrom which are displaced parallel to each other and which periodically scan the monitored region; and a photoelectric light receiver arrangement which receives the light passing through the monitored region and transmits a corresponding electrical signal.
In such known light curtains a mirror wheel is generally provided which periodically deflects an incident light beam over a predetermined angular range and thus ensures the continuous scanning of the monitored region. A disadvantage of such light curtain apparatus is the requirement for a mirror wheel which rotates at high speed or for a rotating or oscillating mirror.
A light curtain apparatus with a light transmitter is already known from DE-PS No. 28 24 311 which consists of ten light emitting diodes arranged in a row alongside one another. The light emitting diodes of the light transmitter are so cyclically controlled by an electrical circuit that they are energised one after the other to transmit flashes of light. The light flashes pass essentially parallel to one another through a monitored region and impinge on a receiving concave mirror at the end of the monitored region. In this arrangement a photoelectric receiver is arranged behind a slot diaphragm at the focal point of the receiving concave mirror.
In this arrangement the concave mirror however merely serves as the receiver while a relatively long luminous diode row is necessary for the cyclical generation of the scanning beam.
If one were to reverse the transmitting and receiving optical systems in this known light curtain then the light would be transmitted from a light source arranged at the focal point of the concave mirror whereas individual receiving collimators arranged in a line would receive beams of light transmitted in bundled beam paths from the concave mirror, and would concentrate them onto a photoelectric converter, which would thereby be cyclically operated.
An optoelectronic monitoring apparatus is furthermore known from DE-PS No. 29 34 554 in which, to generate a scanning beam, a laser light beam is deflected by means of a periodically operating light deflecting device onto a receiving device which includes a strip-like Fresnel lens with a photoelectric receiver arranged at the focal point thereof. In this arrangement it is again disadvantageous that a mechanically operating light deflecting device has to be provided.
Finally, a light barrier grid is known from DE-PS No. 22 47 053 which includes a plurality of light emitting diodes of which each is arranged in the focal point of its own transmitting lens.
In this arrangement the transmitting lenses are arranged closely alongside one another, so that closely adjacent parallel monitoring light beams result.